This invention relates to telephone systems for transmission of both voice and data and, more particularly, to a system wherein all telephone sites are interconnected by a common coaxial transmission line with switching and routing functions being performed by on-site control units which couple the telephones to the transmission line.
Presently, there are two basic types of telephone and data switching systems in use. The first type of system, which is the most widely used, incorporates central office switching. Such systems include the private branch exchange (PBX) which operates in the manner of a miniature central office. The second type of system is the local area network (LAN) which employs digitized voice and data transmission over a common cable.
The central office or PBX type of system is characterized by the use of individual wires, such as a pair of wires or group of such pairs, to connect each telephone instrument to a central switch. The overall configuration of such a system has the form of a star with the central office switching at the middle and the individual telephone instruments being connected to the central office by their respective telephone lines. The central switch and its control equipment performs all of the necessary management functions for providing the voice and data communications, thereby to allow individual telephone instruments to communicate with any other one of the telephone instruments. In recent years, the telephone management functions, whether in a central office or in an on-site PBX, have been expanded to include a variety of sophisticated capabilities such as call forwarding, call camp-on, conference calling, call back, auto dial, and least-cost routing as well as providing statistical information for efficient management of the system.
The major disadvantages of the central switching approach arise from the dependence on the star wiring configuration, this resulting in an excessively large size and complexity to the central switch for the interconnection of thousands of telephone instruments. The star wiring configuration is not economical for small telephone systems. Furthermore, the star wiring configuration requires very large amounts of wire because the telephone line must extend from the location of each telephone instrument all the way to the central switch. Changes in the instrument may require new wiring as is the case wherein a call director instrument is substituted for a single line instrument necessitating the installation of multiple telephone lines. If the original single line instrument were re-installed, the surplus wire would be useless.
Other disadvantages arise with the star wiring configuration in that dependence on the central switch causes system fragility because a switch failure shuts down the entire system. If more advanced functions are to be accommodated with the central switch, the switching equipment must contain a complex and powerful computer and very sophisticated software to manage the thousands of instruments simultaneously, as well as to switch voice traffic among them. Such complexity further increases the fragility of the central office switch.
The LAN incorporates a number of interconnected on-site PBX's via a common cable capable of high speed data transmission. Each PBX is substantially smaller than the usual central office switch. The resulting system has the form of clusters of star-wired PBX units connected to each other by a common cable network which may be configured in either the shape of a ring or a star configuration. While such a system may employ somewhat less wire than a central office system accommodating the same number of instruments, the system suffers, essentially, from the same disadvantages as the aforementioned central office system. Although each PBX unit is smaller and therefore less complex than the central office system, the PBX unit must be provided with the necessary functions to permit the unit to coordinate and connect with other ones of the PBX units.
Another approach to the design of a LAN has been the use of a common cable to physically connect individual instruments or data terminals. Significantly, these systems have all relied on digital signal processing implementations to allow the multiplexing of both voice and data signals in high speed transmission bands of the common cable. Such digital signal processing has limited the practical capacity of the cable to a few hundred simultaneous conversations, in contrast with a desired capacity of carrying tens of thousands of conversations. The poor utilization of the digital approach results in such impracticality that the LAN's have been used primarily to carry high speed data traffic instead of voice communication.
Attempts have been made to enlarge the signal-carrying capacity of the cable by frequency division wherein plural transmission bands are provided to carry digital signals with the bands being spaced apart along the frequency spectrum. Such frequency division adds complexity to the coordinating and the connecting of communications among the plural transmission bands, thereby rendering such a system to be impractical for voice transmission. Additionally, broadband telephone transmission devices have heretofore required manual adjustment of their transmitting power levels to insure that all signals received on the common cable are of substantially the same strength. While the deployment of large numbers of such broadband devices would be desirable in the use of a telephone system, the manual adjustment of power levels would render such deployment impractical.